The invention is concerned with the field of turbomachines and relates to a device for injecting a mixture of air and fuel into a combustion chamber.
It relates more specifically to a novel configuration of an injection device intended to be mounted within a restricted space of a combustion chamber.
Throughout the remainder of the description, the terms “upstream” or “downstream” will be used to denote the positions of the structural elements with respect to one another in the axial direction, taking the gas flow direction as reference. Likewise, the terms “internal” or “radially internal” and “external” or “radially external” will be used to denote the positions of the structural elements with respect to one another in the radial direction, taking the axis of rotation of the turbomachine or the axis of symmetry of the structure in question as reference.
A turbomachine comprises one or more compressors delivering pressurized air to a combustion chamber in which the air is mixed with fuel and ignited so as to generate hot combustion gases. These gases flow in the downstream direction of the chamber toward one or more turbines which convert the energy thus received in order to rotate the compressor or compressors and provide the necessary work, for example, to power an aircraft.
Typically, a combustion chamber used in aeronautics comprises an internal wall and an external wall interconnected at their upstream end by a chamber end wall. The chamber end wall has, spaced circumferentially, a plurality of openings each accommodating an injection device at whose centre is positioned an injector, the assembly allowing the mixture of air and fuel to be fed into the chamber.
The combustion chamber is supplied by liquid fuel mixed with air from a compressor. The liquid fuel is fed right to the chamber by the injectors by means of which it is vaporized into fine droplets. This vaporization is initiated in the region of the injector and is continued particularly in the region of the venturi and the bowl by the effect of pressurized air from the compressor. This pressurized air passes, on the one hand, through the radial swirler or swirlers of the injection device so as to cause the fuel sprayed by the injector to rotate, and, on the other hand, through orifices formed in various parts of the injection device, such as the bowl.
As illustrated particularly in document FR 2 753 779, an injection device has a symmetry of revolution and comprises, arranged from upstream to downstream, a sliding bushing connected by an annular cup to an internal radial swirler. The internal radial swirler is itself connected to an external radial swirler by a venturi, the internal and external radial swirlers being co-rotating, that is to say that the air injected into the injection device by these two swirlers has the same direction of rotation to the axis of symmetry of the injection device. The external radial swirler is then connected by its downstream end to a bowl with a divergent conical wall. The bowl is itself connected to the chamber end wall via a deflector.
Such an injection device has a relatively large outside diameter, and therefore a relatively large bulk, which is not able to be reduced on account of the presence of certain elements such as the radial swirlers. Given its relatively large dimensions, this type of device is suited to receiving an injector of the aeromechanical type.
There are two types of injectors: aerodynamic injectors and aeromechanical injectors.
Aerodynamic injectors make it possible to inject fuel at a pressure close to that of the air which is injected at the injection device. Only one fuel supply circuit is necessary to cover the entire fuel flow range that has to be covered by the injector.
In aeromechanical injectors, the fuel is injected at a much greater pressure than the combustion chamber pressure. The vaporization of the fuel is of good quality, including when the pressure in the chamber is low. To cover the entire fuel flow range while maintaining an acceptable fuel pressure, two fuel supply circuits are necessary, resulting in an injector outside diameter which is larger than in the case of an aerodynamic injector. In spite of this larger bulk, aeromechanical injectors make it possible, inter alia, to improve fuel spraying and also combustion at a low flow rate.
If the available space for housing the injection device is small, for example in the case of smaller turbomachines, the injection device according to the prior art cannot be used because of its excessively large outside diameter. It is therefore necessary to find a novel injection device configuration that ensures at least as good a spraying quality as in the prior art and that can be housed within a small space, that is to say comply with a constraint on its outside diameter.
Moreover, since an aeromechanical injector has certain advantages, it may be advantageous or even necessary to keep this type of injector, which implies that the injection device must comply with an additional constraint on its inside diameter, this inside diameter having to be sufficiently large for an aerodynamic injector to be arranged therein.
Since the radial bulk available for the injection device is thus reduced, it is also necessary to have an effective cross section, for the flow of pressurized air through the various orifices in the injection device, equivalent to that of the prior art in terms of the effect on vaporization, so as to maintain a good spraying quality.